In recent years, the densification of mounted circuit elements is progressing with the demand of downsizing of electronic equipment and of improvement in signal speed and capacity. However, it is becoming a problem that electrical noise increase causes data error. In order to suppress generating of this electrical noise and to stably operate a semiconductor device, it is important to supply a necessary current from a portion near the semiconductor device. For that, it is effective to arrange a capacitor with a large capacity as a decoupling capacitor directly under the semiconductor device.
Here, as a method of arranging a capacitor to a printed wiring board, there is also a method of arranging external capacitors, such as a chip capacitor to the printed wiring board. However, in respect of downsizing, it is advantageous that an inorganic filler is added to the inner layer of the printed wiring board, to thereby give a capacitor ability to the printed wiring board itself, and a method (JP-A-57852/1993 and JP-A-85413/1994) using a composite in which the inorganic filler and resin are mixed as an interlayer insulation material has been known. However, the relative dielectric constant of the composite obtained by the above-mentioned method was as low as about 10 to 20.
Although it is possible to increase the relative dielectric constant of the composite dielectric material containing the inorganic filler by increasing the addition of the inorganic filler, there is a problem that the relative dielectric constant does not increase even if the content of the inorganic filler increase, when the content of the inorganic filler exceeds 50 volume %. Furthermore, since its viscosity becomes too high if an inorganic filler having a high dielectric constant is mixed to a resin too much, a large quantity of solvent is usually needed.
The conventional high dielectric constant composition has been made by removing solvent from and solidifying the paste composition containing an inorganic filler, a resin, and a solvent (JP-A-158472/1998). However, when the content of solvent is large, faults such as decreasing heat resistance due to residual solvent and generating voids around its surface were brought about.
As an method of achieving a high relative dielectric constant, it is known to add a filler having two or more kinds of particle size to increase filling factor, to thereby make relative dielectric constant high (JP-A-88198/1978, JP-A-233669/2001). However, because the filler used therein has a mean particle diameter of the filler of 5 μm or larger as the greatest mean particle diameter and this filler had to be mixed with a resin, the thickness of the composite obtained could not be other than as thick as about 300 μm.
On the other hand, there is a technique using an inorganic filler with large particle diameter as a method of making the dielectric constant high. The dielectric constant of a filler depends on the crystal structure of the filler. Generally speaking, concerning inorganic crystal, as seen such as in barium titanate, the mismatch of center-of-gravities between the anion and the cation brings about a large dielectric constant. If the particle diameter of filler becomes small, generally saying, crystal grain size also becomes small and the surface energy of the particle becomes large, and the symmetricalness of the crystal structure increases in order to reduce energy of the whole system. If the symmetricalness of the crystal structure increases, because the mismatch between center-of-gravities of the anion and the cation becomes small, a dielectric constant becomes small. Therefore, the dielectric constant can be increased by using a filler with large particle diameter. This effect is remarkable especially in barium titanate. For example, there is an example (JP-A-293429/1996) in which barium titanate of 15 μm of mean particle diameters is used as a filler, and ethyl carbitol (the boiling point is 202° C.) is used as a solvent. However, since the particle size of the filler is large and the specific surface area of the filler is small, even if a solvent having high boiling point is used, removing solvent by heating can be carried out relatively in a short time and at low temperature. Then, solvent removal breaks out at a rate quicker than migration of the resin and the filler accompanied by contraction of the whole system, many voids generates. Generating voids causes a decrease of the dielectric constant. When a filler with a large mean particle diameter is used, although the dielectric constant of the filler itself becomes large, generating of voids can not be controlled as mentioned above even if a solvent having a high boiling point is used, and as a result, the dielectric constant was 52 and was not able to acquire a large value. Furthermore, since the filler of the large mean particle diameter of 15 μm is used (JP-A-293429/1996), the thickness had to be large as 25 μm, therefore the density of capacitance is as small as 1.8 nF/cm2.
On the other hand, in order to make system to be mounted in the interior small and thin, a high density SiP (system in package) equipped with LSI with not only memory LSI but also LSI with many terminals is being rapidly developed, however, the capacitor build in this SiP is required strongly to be thin so that the thickness of this interlayer insulation material for capacitors to be 10 μm or thinner. However, by the conventional technique, the demand of making the thickness of 10 μm or thinner cannot be satisfied, and it cannot respond to the needs for making the thickness of the interlayer material thinner which has been rapidly increased in making the performances of mobile devices higher, such as a cellular phone.
Furthermore, since the capacitance of a capacitor is in inverse proportion to the thickness of interlayer insulation material, in view of increasing capacitance of a capacitor, it is also preferable to make the thickness of the interlayer insulation material thinner.
Furthermore, a low coefficient of linear expansion is an important basic property required in the interlayer insulation material. The coefficient of linear expansion of resin itself is 50 ppm/° C. or larger, and is very large as compared with the coefficient of linear expansion of the metal used as a wiring layer, for example, copper (17 ppm/° C.). Therefore, when an interlayer insulation material which consists only of resin is used, a fault by stress, such as an interlayer delamination and a disconnection of wiring arise due to the difference of coefficient of linear expansion with a wiring layer. On the other hand, since the coefficient of linear expansion can be made low if the resin and the inorganic filler are made into a composite, when the composite in which the inorganic filler and the resin are mixed is used as an interlayer insulation material, it becomes possible to bring the value of the coefficient of linear expansion close to that of the wiring layer. However, by the conventional method, since an inorganic filler could not be filled into sufficiently high filling factor, it was not able to lower the value the coefficient of linear expansion of the interlayer insulation material to almost near that of the wiring layer.